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Nachdruck verboten. 



On the Primitive Tetrapod Limb. 



By D. M. S. Watson, M.SC, 



Lecturer in Vertebrate Palaeontology in University College London. 

 With two figures. 



One of the most curious and most constant features of the mam- 

 malian skeleton is the presence of a navicular in the tarsus. This bone 

 is, as has long been recognised, a centrale; and its peculiarity lies in 

 the fact that its tibial side forms part of the internal border of the 

 tarsus; which is thus three rowed on the tibial whilst only two rowed 

 on the fibular side. 



This very characteristic type of tarsus also occurs in all Ther- 

 apsids in which it is known, Dimetrodon, Dicynodon, Galechirus. 

 In embryo Didelphis there are in one stage two centralia taking the 

 place of the navicular and the same condition occurs amongst reptiles 

 in a smallPermian type from South Africa, nearly allied to if not identical 

 with the American Areoscelis, which is perhaps to be associated with 

 the ancestory of the lizards. In Stegocephalia three centralia may 

 be found having the same position as in Archegosaurus and Tre- 

 matops ? 



In all these types, no matter how many centralia there may be, 

 the fourth and fifth distal tarsals are in direct contact with the proxi- 

 mal row, usually entirely with the fibulare. 



In the amphibia there are three bones in the proximal row, the 

 fibulare articulating with the fibula, the intermedium with both 

 the fibula and tibia, and the tibiale with the tibia. In reptiles and 

 mammals the intermedium fuses with the tibiale to form the astra- 

 gulus. 



This type of tarsus with eleven bones is obviously the most 

 primitive known amongst tetrapods, for from it any other type can 

 be readily derived by reduction, whilst it cannot be derived from any 

 other type because all others have lost bones, and in accordance with 

 Dollos dictum of the Irreversibility of Evolution, organs once lost 

 cannot be regained. 



